Estrogens are an important class of steroidal hormones that stimulate the development and maintenance of fundamental sexual characteristics in humans. In the past, estrogens have been found useful in the treatment of certain medical conditions and diseases. For example, estradiol, a steroid hormone produced by the ovary, is useful in the treatment of osteoporosis, cardiovascular disease, premenstrual syndrome, vasomotor symptoms associated with menopause, atrophic vagginitis, Kraurosis vulvae, female hypogonadism, primary ovarian failure, excessive hair growth and prostatic cancer. Unfortunately, administration of such steroids have been associated with a number of side effects, including myocardial infarction, thromboembolism, cerebrovascular disease, and endometrial carcinoma.
For example, hormone replacement therapy (HRT) with estrogen has been determined to be a clinically effective treatment for osteoporosis in post-menopausal women, however less than 15% of eligible women are currently prescribed HRT despite clinical trials that have demonstrated a 50% reduction in hip fractures and a 30% reduction in cardiovascular disease. Non-compliance arises from patient and physician concerns over the two fold increased risk of endometrial cancer observed with HRT employing estrogen alone as well as the association between estrogen therapy and breast cancer. Although unproven in the clinic, this suspected risk for breast cancer has led to HRT being contra-indicated in a significant percentage of post-menopausal women. Co-therapy with progestins has been shown to protect the uterus against cancer while maintaining the osteoprotective effects of the estrogen, however the progestin introduces other side-effects such as withdrawal bleeding, breast pain and mood swings.
In light of problems associated with estrogen therapy, a significant amount of research has been carried out to identify effective nonsteroidal estrogen and antiestrogenic compounds. In general, such compounds may be characterized as both estrogenic and antiestrogenic because while they all bind to the estrogen receptor, they may induce an estrogenic or antiestrogenic effect depending upon the location of the receptor. In the past, it has been postulated that the binding of various nonsteroidal estrogen and antiestrogenic compounds to the estrogen receptor was due to the presence of a common pharmacophore (shown below in Scheme A) which was recurrent in the chemical structures of these compounds. ##STR1## This pharmacophore later became the structural backbone around which nonsteroidal estrogen and antiestrogenic compounds were constructed. Its presence in the constructs of various compounds such as hexestrol, tamoxifen, chroman, triphenylethylene, DES, clomiphene, centchroman, nafoxidene, trioxifene, toremifene, zindoxifene, raloxifene, droloxifene, DABP, TAT-59 and other structurally related compounds has become accepted in the art as the molecular key to estrogen receptor binding specificity.
An example of one noteworthy nonsteroidal antiestrogen is tamoxifen (TAM), (Z)-1,2-diphenyl-1-4-2-(dimethylamino)ethoxy!phenyl!-1-butene, which is a triphenylethylene derivative. Tamoxifen effectively antagonizes the growth-promoting effect of estrogens in primary target tissues such as the breast and ovum.
Currently, this non-steroidal estrogen as well as a structurally similar compound known as raloxifene have been developed for the treatment and/or prevention of osteoporosis, cardiovascular disease and breast cancer in addition to the treatment and/or prevention of a variety of other disease states. Both compounds have been shown to exhibit an osteoprotective effect on bone mineral density combined with a positive effect on plasma cholesterol levels and a greatly reduced incidence of breast and uterine cancer. Unfortunately, tamoxifen and raloxifene both have unacceptable levels of life-threatening side effects such as endometrial cancer and hepatocellular carcinoma.
Accordingly, it would be advantageous to develop a series of non-steroidal compounds which retain beneficial characteristics such as osteoprotective activity while minimizing any undesirable side effects. While it is presently accepted that the pharmacophore backbone mentioned above is responsible for estrogen receptor binding specificity, it has now been discovered that certain novel estrogen binding ligands can be constructed as set forth herein which incorporate particular moieties onto such pharmacophore-based compounds, thereby maximizing beneficial characteristics such as osteoprotective function while minimizing undesireable characteristics such as an increased risk of cancer.